The history of electrochemical energy storage devices, especially capacitors and batteries, has involved attempts to reduce package size while increasing the electrical energy storage capacity. Recent advances in battery and capacitor design have increased life, efficiency and energy density. However, although many of the devices embracing the recent technological advances have filled a need, there continues to be a requirement for efficient, high power density electrical storage devices which withstand the rigors of continuous use and virtually unlimited cycling.
Both capacitors and batteries store energy by the separation of positive and negative charges. The need to store greater amounts of energy in a smaller package continues to drive new research into complex and esoteric ways of making storage devices. The result is that many batteries and capacitors have become expensive and complicated, requiring sophisticated charging schemes and fabrication methods. For example, pseudocapacitors require electrodes made from expensive rare earth materials, and some capacitors and batteries utilize toxic or hazardous chemicals to achieve high charge density. Clearly, a need exists for an electrical energy storage device that combines the desirable features of conventional electrochemical batteries and conventional capacitors, yet can store much larger amounts of energy in a smaller package and can be manufactured at a reasonable cost.